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Expanding 7 Wire Strand Use

Hello,

I was wondering if there is a sprinkler controller or an added attachment to a controller that would allow the expansion of use of a 7 control wire strand bundle. With a 7 wire bundle I can control 6 valves using the conventional method of 1 wire acting as a common to all valves and then each of the 6 remaining wires being unique to each valve. Problem is I want to expand my sprinkler system without having to run additional wire from the current valve terminus to the controller. I've been thinking about this and it seems that one could expand the use of the 7 wire strand with the use of a controller that would control the sprinkler valves by a unique combination of 2 wires that service each valve instead of making 1 wire common to all valves. From my calculation I could expand the use of a 7 wire strand from 6 valves to 21 valves with just the controller supplying the unique combination of power to the 2 wires that service each valve I want to operate and no decoder necessary at the valves. In other words the following shows the combinations of wire use using the conventional method and an expanded use method:

The major drawback that I see is that it may be hard to operate more than 2 valves at once depending on which combination of wires are being used, but realistically operating 2 or more valves at once is probably going to reduce water pressure and something I don't do anyways.

Does anybody know of any additional problems this would present? Are there any products on the market that would allow the expanded use of 7 strand wire?

From the current valves to the controller I would estimate about 50 feet. I ran the original 7 wire bundle in 1/2 inch gray pvc wire conduit and would like to keep it that way. Adding 2 more 7 wire bundles to a 1/2 inch conduit would not work I don't think. So, in order to do that I would have to replace the 1/2 inch with 3/4 inch, go through landscaping that is more filled in now, and under a sidewalk. With the expansion the furthest valve would probably be about 250 feet away from the controller. The biggest issue is trying to avoid the hassle of laying new wire out to the current valves.